Enhance RTCUIApplicationStatusObserver thread safety.

webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m

Index: webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m
diff --git a/webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m b/webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m
index 1d2aab48e852945fe25bd9d08a66ebb2fde9a0ac..7134773610b857304ca767662672937df4249127 100644
--- a/webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m
+++ b/webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.m
@@ -26,6 +26,7 @@
 @implementation RTCUIApplicationStatusObserver {
   BOOL _initialized;
   dispatch_block_t _initializeBlock;
+  dispatch_semaphore_t _waitForInitializeSemaphore;
   UIApplicationState _state;
 
   id<NSObject> _activeObserver;
@@ -45,6 +46,12 @@
   return sharedInstance;
 }
 
+// Method to make sure observers are added and the initialization block is
+// scheduled to run on the main queue.
++ (void)prepareForUse {

[tkchin_webrtc, 2017/09/13 18:32:02]

This class feels more complex than it needs to be.
I suspect that if we constructed multiple instances instead of a singleton,
created the instance in encoder / decoder ctors, provided an isReady method,
ignored frames until main queue initialization is complete, we'd have the same
thing working. I'd guess there'd be sufficient time between encoder / decoder
ctor and the first encode/decode method call to complete init in most cases
because it will take time for capture to begin.

[andersc, 2017/09/14 08:08:45]

I agree, and I mentioned a solution similar to that in b/65558688#comment4

The reason I did it this way was that it is a logically smaller change than from
what we had before, and I'm not confident that returning OK from the
encoder/decoder without performing the work doesn't impact any other record
keeping made by WebRTC. But it should be easy to try out and see if it works as
expected. Let me know if you think we should spend time on trying such a
solution out instead.

+  __unused RTCUIApplicationStatusObserver *observer = [self sharedInstance];
+}
+
 - (id)init {
   if (self = [super init]) {
     NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
@@ -65,6 +72,7 @@
                                                 [UIApplication sharedApplication].applicationState;
                                           }];
 
+    _waitForInitializeSemaphore = dispatch_semaphore_create(1);
     _initialized = NO;
     _initializeBlock = dispatch_block_create(DISPATCH_BLOCK_INHERIT_QOS_CLASS, ^{
       weakSelf.state = [UIApplication sharedApplication].applicationState;
@@ -84,10 +92,19 @@
 }
 
 - (BOOL)isApplicationActive {
+  // NOTE: The function `dispatch_block_wait` can only legally be called once.
+  // Because of this, if several threads call the `isApplicationActive` method before
+  // the `_initializeBlock` has been executed, instead of multiple threads calling
+  // `dispatch_block_wait`, the other threads need to wait for the first waiting thread
+  // instead.
   if (!_initialized) {
-    long ret = dispatch_block_wait(_initializeBlock,
-                                   dispatch_time(DISPATCH_TIME_NOW, 10.0 * NSEC_PER_SEC));
-    RTC_DCHECK_EQ(ret, 0);
+    dispatch_semaphore_wait(_waitForInitializeSemaphore, DISPATCH_TIME_FOREVER);
+    if (!_initialized) {
+      long ret = dispatch_block_wait(_initializeBlock,
+                                     dispatch_time(DISPATCH_TIME_NOW, 10.0 * NSEC_PER_SEC));

[tkchin_webrtc, 2017/09/13 18:32:02]

What's the benefit of setting a short timeout here? It seems to introduce bad
behavior if the timeout isn't met, and it seems hard to guarantee that this
number works for all iOS hardware. DISPATCH_TIME_FOREVER seems more appropriate.

[andersc, 2017/09/14 08:08:46]

10 seconds was picked to be a long timeout. A test deadlocked (because the main
queue was blocked on something else), so added the DCHECK to make it easier to
debug similar issues in the future, rather than the test just hanging.

+      RTC_DCHECK_EQ(ret, 0);
+    }
+    dispatch_semaphore_signal(_waitForInitializeSemaphore);
   }
   return _state == UIApplicationStateActive;
 }